<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@3.3.7/dist/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
    <style type="text/css">
        #bg{
            position: fixed;
            width: 100%;
            height: 100%;
        }
    </style>
</head>
<body>
     <img  id="bg"  src="img/pingmian.gif">
</body>
</html>